1. Field of the Invention
The invention relates to a color image signal processing device for converting a color signal of an image for a first device, such as a color CRT, to a color signal with which the image is reproduced by a second device, such as a color printer.
2. Description of Related Art
In a conventional color image signal processing device for converting RGB (red, green and black) signals for a first device, such as a color CRT, to CMYK (cyan, magenta, yellow, black) signals which are control signals for a second device, such as a color printer, a conversion process is generally carried out as follows when the color signal is described with 8 bits in each device. That is, assuming that the three signals of CMY are complementary color signals of RGB, the conversion processing is carried out according to the following equation: EQU C=255-R, M=255-G, Y=255-B
Various calculation methods have been proposed to calculate the black color signal of K and, generally the minimum value of each signal of CMY is used.
However, in such a color image signal processing device using numerical values which are dependent on the respective devices, it is difficult to match a reproduced color image with user's color sense, and it is also difficult to perform color correction and color conversion so as to achieve visual color matching.
Further, a color-correction and color-conversion target mainly concentrates on the adjustment of hue. For the color signals of the RGB and CMYK, the color to be reproduced is dependent on a device, so that conversion to accurate hue data cannot be performed and accurate transmission cannot be performed.
In order to solve this problem, some proposals have been made in U.S. Pat. Nos. 4,965,664 and No. 4,843,573. In these proposals, from the viewpoint that most of color-correction targets are concentrated on the correction of hue, the color correction is performed on the basis of three signals of H (hue), V (brightness) and C (chroma) (not using the RGB signals). Further, the problem that the color reproduction is dependent on device characteristics can be solved by using signals which are based on the CIE (Commission Internationale de l'Eclairage or International Commission on Illumination) system as an international standard which is not dependent on a device. In the CIE system, an XYZ color system (hereinafter, the value of XYZ is referred to as "CIE1931XYZ value") was instituted as a method of representing a color with a numerical value in 1931. A L*a*b* color system (hereinafter, the value of L*a*b* is referred to as "CIE1976L*a*b* value") was instituted in 1976.
However, according to these conventional methods, the color matching is performed only on the basis of the color values of the CIE system and, thus, it is not applicable to overcome a visual color difference due to the difference in the color mixing mechanism between the color reproduction of an additive color mixing system based on coloring like CRT and the color reproduction of a subtractive color mixing system based on ink color materials like a printer. Furthermore, in some cases it is better to reproduce a color different from an actual color, for colors such as skin color, sky color, or lawn color (so-called memory color), to achieve excellent color reproduction. However, the conventional color conversion method aims to achieve numerically coincident color at all times. Thus, in some cases it does meet the user's intention.